The present invention relates to a ball screw.
The ball screw is directed to a linear guide apparatus composed of a screw shaft, a nut and balls. Helical grooves are defined in an outer peripheral surface of the screw shaft and in an inner peripheral surface of the nut, and these helical grooves form a raceway of balls. When balls roll in the raceway, the nut linearly moves with respect to the screw shaft. That is, the helical grooves of the screw shaft and the nut form the ball rolling grooves. In addition, the nut is provided at the outside with a circulating path.
Ball screws as mentioned above have been demanded to reduce vibrations and noises. For example, Japanese patent examined publication No. Hei. 7-9259, Japanese utility model examined publication No. Hei. 6-47159 and Japanese Utility Model Registration No. 2,582,068 describe ball screws devised to reduce vibrations and noises caused in the circulating path. Japanese patent unexamined publication No. Hei. 11-51049 describes a ball screw having a retainer of a special structure for smoothly receiving balls between the ball rolling groove and the circulating path.
As a device for reducing vibrations and noises caused in the ball rolling groove, Japanese patent unexamined publication No. Hei. 11-315835 describes a technology. This publication sets forth that separators having ball holding surfaces are arranged between balls and balls to cancel contacting of balls one another.
On the other hand, Japanese Patent No. 2,881,855 describes grinding the ball rolling groove in the ball screw, followed by a super finishing.
However, these prior arts still have rooms for further improving the reduction of vibrations and noises of the ball screw.
It is an object of the present invention to provide a ball screw more effectively reducing vibrations and noises than conventional ball screws.
The above object can be achieved by a ball screw comprising:
a screw shaft having a first helical ball rolling groove in an outer periphery thereof;
a nut having a second helical ball rolling groove in an inner periphery thereof; and
a plurality of balls arranged in a helical raceway defined by the first and second helical ball rolling grooves,
wherein at least one of the first and second helical ball rolling grooves has a surface with a surface roughness of 0.12 xcexcm or less in average roughness (Ra) along a helical direction of the helical raceway.
In an embodiment of the present invention, the first helical ball rolling grooves may have the surface with a surface roughness of 0.12 xcexcm or less in average roughness (Ra) along a helical direction of the helical raceway.
In addition, in an embodiment of the present invention, the second helical ball rolling grooves may have the surface with a surface roughness of 0.12 xcexcm or less in average roughness (Ra) along a helical direction of the helical raceway.
Further, in an embodiment of the present invention, both of the first and second helical ball rolling grooves may have the surface with a surface roughness of 0.12 xcexcm or less in average roughness (Ra) along a helical direction of the helical raceway.
Furthermore, in an embodiment of the present invention, the ball screw may further comprises:
a retainer rotatably supporting the balls between the screw shaft and the nut, wherein the retainer may comprise a plurality of separators interposed between adjacent balls, each of the separators having concave surfaces on both sides thereof.
Moreover, in an embodiment of the present invention, the ball screw may further comprises:
a retainer rotatably supporting the balls between the screw shaft and the nut,
wherein the retainer may comprise a plurality of pockets for rotatably holding the balls respectively, each of pockets having concave surfaces between which each of said balls is interposed.
In a preferable embodiment of the present invention, each of the balls may have an outer diameter of 3.969 mm or less.
Further, in the preferable embodiment of the present invention, each of the balls may have an outer diameter in the range from 2.000 mm to 3,969 mm with both inclusive.
As the finishing methods for making the surface roughness of the ball rolling groove 0.12 xcexcm or less in the average roughness (Ra), there are a super finish grinding, a horning finish, a lapping finish, polishing finish or buffing finish.
Moreover, in a preferable embodiment of the present invention, the ball screw may further comprise a retainer rotatably supporting the balls between the screw shaft and the nut,